Subsonic might be useful if your subs don't like the low bass notes..the box couldn't care less but obviously you would be restricting what it's capable of. The frequency also depends on what your subs are ok with. I personally don't use it on my current setup...well I try not to anyway. M amp has it on whether I like it or not so I just have it set as low as possible. As for the LPF 80hz?? sounds good to me...but can your highs keep up with the frequencies that low?